Final answer:
A colon should be used in this sentence as it introduces an explanation or expansion of the word 'dream'.
Step-by-step explanation:
In this sentence, a colon should be used.
The sentence could be written as: One day I hope to achieve my lifelong dream: traveling to the moon.
A colon is used when introducing a list or an explanation. In this case, the word 'dream' is being expanded upon by the phrase 'traveling to the moon'.
Learn more about Using a colon or a semicolon in a sentence